Chapter 4 Alkanes: Nomenclature, Conformational Analysis, and an Introduction to Synthesis

1. Chapter 4Alkanes: Nomenclature, Conformational Analysis, and an Introduction to Synthesis

2. Shapes of Alkanes • “Straight-chain” alkanes have a zig-zag orientation when they are in their most straight orientation • Straight chain alkanes are also called unbranched alkanes Chapter 4

3. Branched alkanes have at least one carbon which is attached to more than two other carbons Chapter 4

4. Constitutional isomers have different physical properties (melting point, boiling point, densities etc.) • Constitutional isomers have the same molecular formula but different connectivity of atoms Chapter 4

5. The number of constitutional isomers possible for a given molecular formula increases rapidly with the number of carbons Chapter 4

6. IUPAC Nomenclature of Alkanes, Alkyl Halides and Alcohols • Before the end of the 19th century compounds were named using nonsystematic nomenclature • These “common” or “trivial” names were often based on the source of the compound or a physical property • The International Union of Pure and Applied Chemistry (IUPAC) started devising a systematic approach to nomenclature in 1892 • The fundamental principle in devising the system was that each different compound should have a unique unambiguous name • The basis for all IUPAC nomenclature is the set of rules used for naming alkanes Chapter 4

7. Nomenclature of Unbranched Alkanes Chapter 4

8. Nomenclature of Unbranched Alkyl groups • The unbranched alkyl groups are obtained by removing one hydrogen from the alkane and named by replacing the -ane of the corresponding alkane with -yl Chapter 4

9. Nomenclature of Branched-Chain Alkanes (IUPAC) • Locate the longest continuous chain of carbons; this is the parent chain and determines the parent name. • Number the longest chain beginning with the end of the chain nearer the substituent • Designate the location of the substituent • When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain • Substituents are listed alphabetically Chapter 4

10. When two or more substituents are identical, use the prefixes di-, tri-, tetra- etc. • Commas are used to separate numbers from each other • The prefixes are used in alphabetical prioritization • When two chains of equal length compete to be parent, choose the chain with the greatest number of substituents • When branching first occurs at an equal distance from either end of the parent chain, choose the name that gives the lower number at the first point of difference Chapter 4

11. Nomenclature of Branched Alkyl Chains • Two alkyl groups can be derived from propane • Four groups can be derived from the butane isomers Chapter 4

12. The neopentyl group is a common branched alkyl group • Examples Chapter 4

13. Classification of Hydrogen Atoms • Hydrogens take their classification from the carbon they are attached to Chapter 4

14. Nomenclature of Alkyl Halides • In IUPAC nomenclature halides are named as substituents on the parent chain • Halo and alkyl substituents are considered to be of equal ranking • In common nomenclature the simple haloalkanes are named as alkyl halides • Common nomenclature of simple alkyl halides is accepted by IUPAC and still used Chapter 4

15. IUPAC Substitutive Nomenclature • An IUPAC name may have up to 4 features: locants, prefixes, parent compound and suffixes • Numbering generally starts from the end of the chain which is closest to the group named in the suffix • IUPAC Nomenclature of Alcohols • Select the longest chain containing the hydroxyl and change the suffix name of the corresponding parent alkane from -ane to -ol • Number the parent to give the hydroxyl the lowest possible number • The other substituents take their locations accordingly Chapter 4

16. Examples • Common Names of simple alcohols are still often used and are approved by IUPAC Chapter 4

17. Alcohols with two hydroxyls are called diols in IUPAC nomenclature and glycols in common nomenclature Chapter 4

18. Nomenclature of Cycloalkanes • The prefix cyclo- is added to the name of the alkane with the same number of carbons • When one substituent is present it is assumed to be at position one and is not numbered • When two alkyl substituents are present the one with alphabetical priority is given position 1 • Numbering continues to give the other substituent the lowest number • Hydroxyl has higher priority than alkyl and is given position 1 • If a long chain is attached to a ring with fewer carbons, the cycloalkane is considered the substituent Chapter 4

19. Chapter 4

20. Bicyclic compounds • Bicyloalkanes contain 2 fused or bridged rings • The alkane with the same number of total carbons is used as the parent and the prefix bicyclo- is used • The number of carbons in each bridge is included in the middle of the name in square brackets Chapter 4

21. Nomenclature of Alkenes and Cycloalkenes • Alkenes are named by finding the longest chain containing the double bond and changing the name of the corresponding parent alkane from -ane to -ene • The compound is numbered to give one of the alkene carbons the lowest number • The double bond of a cylcoalkene must be in position 1 and 2 Chapter 4

22. Compounds with double bonds and alcohol hydroxyl groups are called alkenols • The hydroxyl is the group with higher priority and must be given the lowest possible number • Two groups which contain double bonds are the vinyl and the allyl groups Chapter 4

23. If two identical groups occur on the same side of the double bond the compound is cis • If they are on opposite sides the compound is trans • Several alkenes have common names which are recognized by IUPAC Chapter 4

24. Physical Properties of Alkanes and Cycloalkanes • Boiling points of unbranched alkanes increase smoothly with number of carbons • Melting points increase in an alternating pattern according to whether the number of carbon atoms in the chain is even or odd Chapter 4

25. Sigma Bonds and Bond Rotation • Ethane has relatively free rotation around the carbon-carbon bond • The staggered conformation has C-H bonds on adjacent carbons as far apart from each other as possible • The drawing to the right is called a Newman projection • The eclipsed conformation has all C-H bonds on adjacent carbons directly on top of each other Chapter 4

26. The potential energy diagram of the conformations of ethane shows that the staggered conformation is more stable than eclipsed by 12 kJ mol-1 Chapter 4

27. Conformational Analysis of Butane • Rotation around C2-C3 of butane gives six important conformations • The gauche conformation is less stable than the anti conformation by 3.8 kJ mol-1 because of repulsive van der Waals forces between the two methyls Chapter 4

28. The Relative Stabilities of Cycloalkanes: Ring Strain • Heats of combustion per CH2 unit reveal cyclohexane has no ring strain and other cycloalkanes have some ring strain Chapter 4

29. The Origin of Ring Strain in Cyclopropane and Cyclobutane : Angle Strain and Tortional Strain • Angle strain is caused by bond angles different from 109.5o • Tortional strain is caused by eclipsing C-H bonds on adjacent carbons • Cyclopropane has both high angle and tortional strain • Cyclobutane has considerable angle strain • It bends to relieve some tortional strain • Cyclopentane has little angle strain in the planar form but bends to relieve some tortional strain Chapter 4

30. Conformations of Cyclohexane • The chair conformation has no ring strain • All bond angles are 109.5o and all C-H bonds are perfectly staggered Chapter 4

31. The boat conformation is less stable because of flagpole interactions and tortional strain along the bottom of the boat • The twist conformation is intermediate in stability between the boat and the chair conformation Chapter 4

32. Substituted Cyclohexanes: Axial and Equatorial Hydrogen Atoms • Axial hydrogens are perpendicular to the average plane of the ring • Equatorial hydrogens lie around the perimeter of the ring • The C-C bonds and equatorial C-H bonds are all drawn in sets of parallel lines • The axial hydrogens are drawn straight up and down Chapter 4

33. Methyl cyclohexane is more stable with the methyl equatorial • An axial methyl has an unfavorable 1,3-diaxial interaction with axial C-H bonds 2 carbons away • A 1,3-diaxial interaction is the equivalent of 2 gauche butane interactions Chapter 4

34. Disubstitued Cycloalkanes • Can exist as pairs of cis-trans stereoisomers • Cis: groups on same side of ring • Trans: groups on opposite side of ring Chapter 4

35. Trans-1,4-dimethylcylohexane prefers a trans-diequatorial conformation Chapter 4

36. Cis-1,4-dimethylcyclohexane exists in an axial-equatorial conformation • A very large tert-butyl group is required to be in the more stable equatorial position Chapter 4

37. Bicyclic and Polycyclic Alkanes • The bicyclic decalin system exists in non-interconvertible cis and trans forms Chapter 4

38. Synthesis of Alkanes and Cycloalkanes • Hydrogenation of Alkenes and Alkynes Chapter 4

39. Reduction of Alkyl Halides Chapter 4

40. Alkylation of Terminal Alkynes • Alkynes can be subsequently hydrogenated to alkanes Chapter 4

41. Retrosynthetic Analysis-Planning Organic Synthesis • The synthetic scheme is formulated working backward from the target molecule to a simple starting material • Often several schemes are possible Chapter 4